Bones provide support for muscle movement and protection for organs. Living bone tissue is in a constant state of flux due to the process of bone remodeling. The bone matrix is continuously deposited and resorbed. This rapid turnover of bone occurs throughout childhood as bones increase in size, density, and thickness until the individual reaches a genetically-determined adult height. Once at adult height, bones cease to grow in size but continue to increase in thickness until the individual reaches approximately 30 years of age. As bone growth ceases, the bone is resorbed faster than it is replaced, thus leading to a gradual thinning of the bones. The thinning bone tissue deteriorates and creates spaces or pores between the units of the bony matrix. This process is commonly referred to as osteoporosis.
Osteoporosis is characterized by increased fragility of bones resulting from the loss of bone tissue from the interior of the bone tissue (cancellous bone tissue). Such bone loss reduces the overall density of the internal bone tissue (osteopenia). Thus, both cancellous bone tissue and the surrounding cortical bone can both easily fracture/collapse from even minimal trauma. For example, when a vertebral body fractures, it collapses, and may allow the spine to deform collapsing forward and also reducing its overall length. Vertebral body collapse/fractures (VCFs) can cause serious side effects with patients suffering acutely from loss of height, severe localized pain from the loss of structural stability or more chronic long term pain and problems from the resulting secondary developing kyphosis or scoliosis deformity, and from the postural deformity or the resulting chronic instability of the fracture or even increased pressure on the spinal nerves, stretching of muscles, tendons, and ligaments by the misshapen spinal deformity. Often the osteoporotic patient experiences decreased mobility leading to an inability to carry out everyday tasks and overall reduction in quality of life. Obviously, preventing bone loss would significantly improve the health, well-being, and functional capabilities of the patient. Osteoporosis or other bone-related diseases and/or defects can also affect long bones. Significant osteopenia and focal bone destruction may also result from other sources, such as, cancer, destructive tumors, infections, radiation treatments, hemangiomas, corticosteroid regimens, etc.
Osteoplasty refers to any surgical procedure or process by which total or partial loss of bone is remedied. Continuing with the vertebral body example, vertebroplasty and KYPHOPLASTY represent two recently developed minimally invasive osteoplastic procedures for repairing and stabilizing vertebral body collapse/fracture (VCFs) by injecting a flowable hardening material (surgical cement system), usually into the interior (medullary) cavity of a fractured or otherwise damaged vertebral body, usually under image-guided control (fluoroscopic, CT Scan, etc.) The cement material solidifies in situ, thereby stabilizing the fracture, which relieves pain and prevents further collapse of the vertebral body. Vertebroplasty involves the percutaneous injection of the flowable cement into the targeted vertebral body via a delivery means (e.g., trocar, needle, lumen or cannula). Significant correction of the spinal collapse and reconstitution of pre-fracture vertebral height as well as kyphosis correction occurs with spinal positioning on the table into a neutral or lordotic posture in preparation for the vertebroplasty procedure in those fractures which are unstable and still mobile.
KYPHOPLASTY is a surgical procedure similar to vertebroplasty which includes attempted restoration of vertebral height by inflation of a balloon within the interior vertebral body creating a cavity prior to injection of the hardening cement material. This attempts to gain better control of the size and shape of the interior cavity and the resultant size and shape of the cement. Unfortunately with KYPHOPLASTY the cement is limited by the low pressure injection into the void created by the balloon deployment. And therefore does not commonly permeate well into the remaining, adjacent osteoporotic bone and the adjacent subchondral region, leaving it susceptible to further fracture collapse later with weight bearing. As discussed above, the cement ostensibly provides stabilization, or internal casting of VCFs. In both of the aforementioned procedures the cement is inserted under variable pressure by mechanical, electrical, or manual insertion means (e.g., pumps). Substantially less pressure being utilized by the KYPHOPLASTY procedure. Unfortunately KYPHOPLASTY procedure requires much more expensive equipment, larger needles, and implants for the technique which are much more complex and involved and time consuming to perform with insertion of an expandable balloon device which creates a cavity to inject the cement into, under less pressure, purportedly with less cement extravasation. The act of balloon inflation also unfortunately can cause the surrounding intact osteoporotic cancellous bone to fracture further as the intended void is created—to fill with the cement. Increased pain is often experienced later as this newly fractured bone may collapse around the cement. Furthermore there is very little if any permeation and penetration of bone cement into the remaining vertebral osteoporotic body or subchondral endplate level to stabilize this already weakened bone and prevent further fracture and collapse later; clearly a significant benefit of the higher injection pressures utilized with the vertebroplasty procedure.
After injection, the surgical cement material solidifies thereby providing support and reinforcement to the collapsing, fractured bone (e.g. vertebral body) internally. This support restores structural stability to the fractured bone and relieves the compression fracture pain of instability, or deformation of the vertebral body and the pain associated with the kyphosis deformity and/or instability of the fracture. These procedures are monitored by an imaging system to identify extravasation (leakage) of the cement material through cracks and/or gaps in the cancellous and/or cortical bone to the area surrounding the vertebral body or into the spinal canal prior to curing. The incidence of cement extravasation is usually higher in vertebroplasty procedures than the comparative rates in KYPHOPLASTY procedures however it has been shown to be only rarely clinically significant. The injection of cement is usually terminated if extravasation is identified or appears imminent by the radiographic imaging used to monitor the cement flow, or adequate vertebral body fill is accomplished. Unfortunately the optimal amount of vertebral body “fill” with cement may not have been achieved by the time extravasation is identified or imminent, and therein lies the objective of this invention, namely to be able to perform the vertebroplasty procedure with a substantially lower rate of cement extravasation, while providing more satisfactory/complete vertebral body fill of cement.
Polymer-based surgical cement systems have been employed for many years in contemporary orthopedics. In particular, polymethylmethacrylate (PMMA) has emerged as one of the most popular biocompatible cement systems. Other systems used for bone reinforcement, such as, allograft and autograft tissue suffer from numerous disadvantages including; the possibility of disease transmission, availability and expense, and variability of the host response regarding union and repair and slower progression to bony healing with bony incorporation and consequential pain relief. More recently, osteogenic substances (BMP's) have been utilized for such purposes to induce healing of fractured bones, as well as to stabilize them acutely, like a cement.
Polymethylmethacrylate cement systems generally comprise two components; a monomer liquid (e.g., methylmethacrylate), and a dry powder component (e.g., polymethylmethacrylate (PMMA)). These two components are mixed together to begin the polymerization process and are placed inside the receiving site or cavity. The polymeric bone cement will then solidify (harden). Polymer-based bone cement is potentially toxic to the patient and extravasation of the cement has been linked to various clinical factors such as pulmonary embolism and compression of adjacent neural structures including the spinal cord occasionally necessitating emergency decompression surgery. Also extravasation through the fractured endplates into the adjacent disc may occur. Vertebral body cement extravasation is never desirable but rarely clinically significant, as has been shown in multiple clinical studies.